WOW. My sister married an Italian last year and with a very restrictive guest list and two wedding parties (one in Germany, and a smaller one in Italy) we totaled about 250 guests. I understand it can easily go up to 300-400 but anything more ist crazy, even by Italian standards ;-) (Just talking about my experience here, not putting down anyone). The wedding cake is quite important in the Italian tradition. I wonder how they will handle the usual photoshoot with the cake with that many guests. It took us over 2 hours for less than 100!
The tradition (at least in the area of Naples, where the family of my brother-in-law comes from) is that around midnight, the cake is cut, the guests will come up to the bridal couple, hand over their present, recieve the bonboniere (give-away) in return, have their picture taken with the couple and the cake and then either eat some cake or take it with them and leave. With 1500 guests this will take the best part of about 3 days ;-)
Seriously though, I think when the guest list is really final and RSVP'd it will probably boil down to about 800-900. Still madness :-)
The next thing to consider that we Europeans will rarley be satisfied with a Wilton standard-serving of cake. Usually, we plan quite a bit more cake per person.
IF you do this at all (and I would SERIOUSLY think about it) I would agree to a nice 4-or 5 tier for the cutting and pictures and they will have to find a solution for the rest of the servings. You are a teenager, this is more than enough of a wedding present!
I did a company anniversary cake 2 years ago that totaled about 2000 servings, but these were flat sheet cakes, not filled, just iced with ganache, all put together about 40.000 square inches, with the company logo and Happy Birthday in laaaaaarge gumpaste letters. I have a small professional bakery and just the baking and rough-icing with ganache took me 3 days straight with no sleep. I had to buy 100 boxes of the correct size for the cakes and we had a van and a large car to deliver them. It netted me a nice amout of money but I will N E V E R E V E R (can not stress this enough) do anything like this again. So think about this really carefully!!